1. Field of the Invention
This invention relates to a printing apparatus driven by a battery and to a method of charging the battery.
2. Description of the Related Art
In a printing apparatus such as a printer or facsimile machine, the energy driving element of a recording head is driven based upon image information that has been transferred thereto, whereby an image comprising a dot pattern is printed on a recording sheet such as paper or a thin plastic substrate. Depending upon the printing technique, a printing apparatus of this kind can be classified into a number of types, such as an ink-jet type, wire-dot type and thermal type.
In a printing apparatus of this kind, a commercial power supply generally is used as the main power source. In a case where the printing apparatus is of the portable, compact type, a dual power supply arrangement is adopted in which power is capable of being supplied by an AC adapter as well as by a battery. However, in the arrangement wherein the printing apparatus is driven by a battery, it is difficult to drive the components of the apparatus when the output voltage of the battery becomes too low owing to a decline in the residual capacity of the battery. For example, if the printing function ceases owing to a decline in battery voltage during the course of a printing operation, all of the printing information received up to this point vanishes. Further, if such a decline in battery voltage occurs in an ink-jet printing apparatus, a situation can develop in which the ink jetting port of the printing head cannot be capped by a cap member, in which case the nozzle of the ink jetting head may become clogged by dried ink.
Accordingly, in a case where a printing apparatus, especially an ink-jet printing apparatus, is driven by a battery, it is necessary to monitor the capacity of the battery and take some countermeasures when the battery capacity falls below a predetermined value. Generally, in an electronic device driven by a battery, a widely employed technique is to make use of a discharge characteristic in which battery voltage declines with a decrease in battery capacity, with the battery capacity being estimated by detecting the battery voltage. In an ink-jet printing apparatus, the conventional practice is to detect battery voltage and, when the battery voltage falls below a specific voltage, suspend the operation of the apparatus upon determining that the battery capacity is inadequate. The operator is notified of the lack of battery capacity by an indicating element such as a buzzer or lamp.
Generally, in a printing apparatus such as a serial printer, the printing head is mounted on a carriage driven back and forth horizontally by a carriage motor. The recording medium, on the other hand, is conveyed at right angles to the back-and-forth traveling direction of the carriage by conveyor rollers driven by a paper-feed motor. FIG. 8 is a diagram showing the drive timing of the carriage motor and the drive timing of the paper-feed motor. The amount of power consumed during a printing operation is maximum in an interval over which decelerating drive of the carriage motor and accelerating drive of the paper-feed motor overlap, as well as in an interval over which decelerating drive of the paper-feed motor and accelerating drive of the carriage motor overlap (both intervals are indicated by X). The amount of power consumed in these intervals attains a value more than twice that of average power consumption during an ordinary printing operation.
In a case where a battery is employed as the power source, it is required that the printing operation be suspended and that the battery be replaced when battery capacity falls below a predetermined level, even if the battery capacity remaining is comparatively large. The reason for this is to avoid a system-reset operation brought about by a decline of battery voltage in the intervals X. This means that the full capacity of the battery cannot be used, thereby shortening drive time during which the apparatus can be driven between battery exchanges. Accordingly, the only expedient available in order to lengthen drive time by a battery is to use a battery having a larger capacity. Use of such a large-capacity battery not only raises the cost of the apparatus but also leads to an increase in its size and weight and therefore detracts from portability.
A nickel-cadmium (NiCd) battery generally is well known as a large-capacity secondary battery that is capable of being charged. It is known that when a battery of this kind has its final discharge voltage set to a high voltage value of more than 1.1 V/cell and is repeatedly charged and discharged, there is a decline in the discharge capacity or discharge voltage. This phenomenon is known as the "memory effect". This phenomenon will not occur if the battery is charged following discharge to a final discharge voltage of 1.0 V/cell, which is specific to an NiCd battery. In addition, even an NiCd battery in which the memory effect has appeared is capable of being almost fully restored to its original discharge capability. However, in order to protect the conventional printing apparatus before the residual capacity of the battery is completely depleted, operation is terminated automatically when a predetermined voltage value is attained, as mentioned above, and the apparatus cannot be driven unless the battery is charged. Thus, when a comparatively high voltage value is set as the final discharge voltage and the battery is charged when the final discharge voltage is attained, the memory effect develops, there is a decline in the apparent battery capacity and the time during which the apparatus is capable of being driven by the battery shortens further.
A method considered as a countermeasure is to forcibly discharge the remaining capacity of the battery, before charging starts, until the final discharge voltage of the NiCd battery is attained, and then perform charging. With this method, however, a special-purpose discharge circuit for discharging the battery is required. The result is higher cost. Moreover, since such forcible discharging requires 30 minutes to one hour, the total charging time is prolonged.